Cathode ray tube



July 2, 1940. D. w. EPSTEIN 2,206,666

CATHODE RAY TUBE Filed Oct. 22, 1937 1 f fl:

Jmaentor David (Epstein C(ttorneg Patented July 2, 1940 UNITED STATES CATHODE RAY TUBE David W. Epstein, Merchantville, N. J., assignor to Radio Corporation of America, a corporation of Delaware Application ,October 22,

3 Claims.

My invention relates to cathode ray tubes and particularly to tubes of the type in which the cathode ray is electrostatically focused.

It is well known that in a cathode ray tube the cathode ray may be focused to a small spot on a screen by employing cooperating electrodes having suitable voltages applied thereto whereby, due to the resulting electrostatic field, an electron lens is formed having the desired characteristics. Such an electron lens, like an optical lens, has a certain amount of spherical aberration whereby the outline of the spot formed by the cathode ray on the screen may not be as sharp and as small as is desired.

It is, accordingly, an object of my invention to provide a cathode ray tube of the above-mentioned type in which spherical aberration is minimized.

In a preferred embodiment of my invention, the cathode ray tube resembles those of conventional construction except that one or more additional electrodes are positioned between the first anode and the second anode, at least one of the additional electrodes being maintained at voltage having a value intermediate the values of the first and second anode voltages. By properly adjusting the several voltages, the cathode ray is brought to a minimum spot on the screen and there is a minimum amount of spherical aberration.

The invention will be better understood from the following description taken in connection with the accompanying drawing in which Figure 1 is a view of one embodiment of my invention, and

Figure 2 is a view of another embodiment of my invention.

Referring to Fig. l, I have illustrated an elec trostatically focused cathode ray tube of the high vacuum type. It comprises a highly evacuated envelope l comprising a cylindrical portion and a conical portion and having therein an indirectly heated cathode 2, a control electrode 3, a first anode 4, a second anode 6, and a spherical aberration correcting electrode 1. A fluorescent screen 5 is located at the end of the tube.

The electrodes 2, 3 and 4 are of conventional form, preferably being cylindrical in shape. Electrodes 3 and 4 are provided with the usual circular apertures whereby an electron beam of circular cross-section passes therethrough.

The second anode 6 preferably is in the form of a conductive coating on the inner surface of the envelope l although it may be a cylindrical electrode spaced from the wall of the envelope and supported in any suitable manner.

The electrode 1 preferably is cylindrical in shape and is maintained at a potential which is higher than the first anode potential but lower than the second anode potential.

1937, Serial No. 170,327

It should be noted that the tube electrodes are not necessarily cylindrical but they should have symmetry with respect to the tube axis. Such symmetry may be provided by the use of noncylindrical electrodes having circular apertures.

The several tube electrodes may be maintained at the proper potentials by means of any suitable voltage supply such as batteries 8 and 9.

The battery 8 maintains the electrode 3 negative to reduce beam current and to concentrate the electrons in a well known manner as they leave the cathode 2. In the case of a television receiver tube, the electrode 3 also functions to control the intensity of the electron beam in accordance with picture signals supplied through a conductor H and impressed across an input resistor H2.

The battery ll maintains electrodes 4, l and 6 at successively higher potentials. For a particular tube design there is a certain ratio of voltages which reduces the spot size and the spherical aberration to a minimum. If the dimensions and/or the spacings of the electrodes are changed, it will be readily understood that the optimum voltage ratio for reduction of spherical aberration will be different.

The table given below shows how the transverse spherical aberration varied in one particular .tube of the type shown in Fig. 1 when the voltages V4, V7 and V6 were varied, these being the voltages on electrodes 4, l and 6, respectively.

4 V1 Va Aberration AIiZli'meters 300 300 1900 21. 5 300 400 2200 0 300 500 2400 10. 0 300 750 2800 8. 0 300 1000 2900 9. 0 300 1900 1900 17. 0

It will be seen that for this tube the correct voltages for electrodes 4, l and 6 were 300, 750 and 2800, respectively. Of course, if the first anode voltage were made different than 300 volts, the voltages applied to electrodes 6 and 1 should be changed to keep the voltage ratios substantially the same, as the ratios of 300 to 750 to 2800.

It should be noted that the relative diameters of electrodes l, 1 and 6 may be different in specific tube designs. For example, electrode 1 may have a diameter greater than either first anode 4 or second anode 6. Or it may be a metallic coating on the surface of envelope 4 and of the same diameter as the adjacent end of the anode 6. It will be understood that such changes will necessitate changes in the voltages applied to the electrodes.

Regardless of the specific tube structure, the voltages applied to electrodes t, I and B are so adjusted that the cathode ray is focused to the smallest possible spot on the screen structure 5. This generally is the image of the cross-over.

With this adjustment there is the minimum spherical aberration. Also it will be found that the voltage on electrode 1 is always greater than the voltage on electrode 4 and less than the voltage on electrode 6.

It will be understood that my invention applies to cathode ray picture transmitter tubes as well as to receiver or oscillograph tubes. In the transmitter tubes the fluorescent screen 5 is replaced by a suitable screen structure such as a mosaic of light sensitive elements. I

A different ambodiment of my invention is shown in Fig. 2. In Figs. 1 and 2 like parts are indicated by the same reference numerals. It will be seen that the only difierence in the tube structures is that an additional electrode. M has been provided, this electrode being cylindrical in form or otherwise having axial symmetry and being located between electrodes 6 and 7. Thus there are two electrodes positioned between the first and second anodes for reducing spherical aberration. It has been found that at least one of these electrodes should be maintained at a potential greater that that of the first anode and less than that of the second anode. For a specific tube structure, as shown by the table below, the voltages on electrodes 4, 7, I4 and 6 for minimum spherical aberration were 400, 800, 400 and 2600, respectively.

In the following table the voltages V4, V7, V14 and V6 are the voltages on electrodes 4, I, I4 and B, respectively.

Spot diameter Millimeters 400 2. t 400 I claim as my invention: 1. A cathode ray tube of the high vacuum type comprising a cathode, a control electrode, a first anode, a first spherical aberration correcting electrode, a second spherical aberration correcting electrode and a second anode, said cathode, electrodes and anodes being located along the axis of the tube in the order named, means for maintaining said first correcting electrode at a positive potential which is intermediate the potentials of said anodes, and means for maintaining said second correcting electrode at a positive potential which is lower than the potential of said first correcting electrode.

2. In a cathode ray tube, means including a cathode and a first anode for producing an electron beam, and an electron lens comprising said first anode, a first spherical aberration correcting electrode, a second spherical aberration correcting electrode and a second anode for focusing said beam, said first anode, said first and second correcting electrodes and said second anode being located along the axisof said tube in the order named, means for maintaining said first correcting electrode at a positive potential which is intermediate the potentials of said anodes, and means for maintaining said second correcting electrode at at least approximately the potential of said first anode.

3. In a cathode ray tube, means including a cathode and a first anode for producing an electron beam, a second anode and a first spherical aberration, correcting electrode positioned between said first anode and said second anode, a second spherical aberration correcting electrode positioned between said first correcting electrode and said second anode, a screen structure, means for maintaining said anodes and said correcting electrode at positive potentials with respect to said cathode, the potential on said first correcting electrode being greater than that on said first anode and less than that on said second anode, the potential on said second correcting electrode being at least approximately the same asthe potential of said first anode, the relative values of said potentials being such that said beam is focused to a minimum spot on said screen structure.

DAVID w. EPSTEIN. 

